1. Field of the Invention
The present invention relates to a process for producing polyurethane foam having uniform fine cells. A polyurethane foam obtained by the process of the present invention may be suitably used as polishing materials for resins, glass, lens, rock crystals, and silicon for producing semiconductors, electronic substrates, optical substrates etc. Especially a polyurethane foam of the present invention, which is cut as necessary, is suitably used as polishing sheets for CMP.
2. Description of the Related Art
As technology for producing a polyurethane foam, a method in which adding organic solvents having low boiling point, such as fluorocarbons and methylene chloride, into an ingredient composition for foaming and dispersing, and then foaming by polymerization heat vaporization; a method in which adding water into a ingredient composition for foaming and dispersing, and then foaming with carbon dioxide gas generated by a reaction of isocyanate groups and water, are well known. In a foam obtained by these methods, diameter of foamed cells (diameter of cells) has a minimum of 100 μm as an average, it is difficult to obtain a foam having more finely and uniform cells.
Following methods are known as a process for producing a polyurethane foam that has fine cells.    (1) A method that after fine-particles soluble in a solvent are dispersed into polyurethane polymer and are molded into a predetermined form, the molded body is immersed in a solvent that dissolves the fine-particles but does not dissolve the polyurethane polymer, the fine-particles are removed by dissolving to obtain a porous polyurethane resin, that is, polyurethane foam.    (2) A method that finely hollow foamed materials are dispersed in an ingredient composition forming polyurethane.
However, in a case where the above-mentioned method (1) is used, since much amount solvent is required, and treatment for the solvent containing material forming fine-particles is indispensable, high cost naturally occurs. Moreover only open-celled foam may be obtained, the foam obtained may not be used for usage where rigidity is needed, usage is limited. Besides, there is also a problem that a dissolving process and a solvent drying process are also needed, and a long time is needed in order to prepare a very thick molded body.
On the other hand, in the method of (2), since a finely hollow foam has a strong tendency to be floated up in a polyurethane forming solution based on a difference of density, a uniform foam is difficult to be produced. Besides, the finely hollow foam is comparatively expensive, and further since the raw material of the finely hollow foam is remained in the foam, there is a possible problem of inducing damage on a cutting edge in the case where the foam is cut. And the hollows fine-particles are scattered easily and then great costs are needed for facilities of working environmental maintenance.
By the way, a polyurethane foam is applied as a polishing sheet for producing silicon for semiconductors and the like, and producing an electronic substrates. In polishing sheet, highly precise polishing properties are required based on higher density of a formed circuit. Depend on types of particles and size of particles that are contained in polishing slurry used in polishing, a hardness of polishing sheet (polyurethane foam) and the like needs to be matched to those characteristics. For example, ceria derived slurry has a larger particle size than silica derived slurry, and when ceria derived slurry is used as polishing slurry, higher hardness is needed than silica derived slurry.
An object of the present invention is to provide a process for producing a polyurethane foam that have uniform fine cells and higher hardness than ones having the same density can be produced without using a chemically reactive blowing agent such as water, vaporizable expandable blowing agent such as fluorocarbon, or other substance such as a finely particulate hollow foam or solvent soluble substance.
Furthermore, another object of the present invention is to provide a polishing sheet adapted to polishing slurry by using polyurethane foam obtained by the above-mentioned process.